In a SAGD process, steam is injected into a reservoir through a horizontal injection well to develop a vertically enlarging steam chamber. Heated oil and water are produced from the chamber through a horizontal production well which extends in closely spaced and parallel relation to the injection well. The wells are positioned with the injection well directly over the production well or they may be side by side.
SAGD was originally field tested with respect to recovering bitumen from the Athabasca oil sands in the Fort McMurray region of Alberta. This test was conducted at the Underground Test Facility ("UTF") of the present assignee. The process, as practiced, involved:
completing a pair of horizontal wells in vertically spaced apart, parallel, co-extensive relationship near the bottom of the reservoir; PA1 starting up by circulating steam through both wells at the same time to create hot elements which functioned to slowly heat the span of formation between the wells by heat conductance, until the viscous bitumen in the span was heated and mobilized and could be displaced by steam injection to the production well, thereby establishing fluid communication from the developing chamber down to the production well; and PA1 then injecting steam through the upper well and producing heated bitumen and condensate water through the lower well. The steam rose in the developing bitumen-depleted steam chamber, heated cold bitumen at the peripheral surface of the chamber and condensed, with the result that heated bitumen and condensate water drained, moved through the interwell span and were produced through the production well. PA1 that provision of an oil-wet oil membrane at the production end of a column of oil-saturated, water-wet sand was beneficial to recovery; PA1 that the pumping shut-downs or cyclic injection provided quiescent periods during which we postulated that oil was drawn by capillary effects or imbibed into the oil-wet membrane with corresponding displacement of resident water; and PA1 that this combination of features enabled oil to flow more easily through the production end, leading to improved oil production rate and recovery.
This process, as practised at the UTF, is described in greater detail in Canadian patent 2,096,999.
Successful recovery of bitumen during the SAGD process depends upon the efficient drainage of the mobilized bitumen from the produced zone to the production well.
One object of the present invention is to achieve improved drainage, as evidenced by increased oil recovery.